Some article websites (I’m looking at msn.com right now, as an example) show the first page or so of article content and then have a “Continue Reading” button, which you must click to see the rest of the article. This seems so ridiculous, from a UX perspective–I know how to scroll down to continue reading, so why hide the text and make me click a button, then have me scroll? Why has this become a fairly common practice?
Also, a lot of websites are built on CMS that has [Read More]… baked in. eg wordpress is designed around the concept of an excerpt of each page/post as it was built 30 years ago. Although as others have pointed out, the time/data savings are minimal - that mattered when wordpress was invented and is a vestigial part of the system.